Processes are already known for the production of potassium magnesium phosphate, according to which potassium dihydrogen phosphate is reacted with magnesium oxide or magnesium carbonate in the amount of water which is bound by the potassium magnesium phosphate as water of crystallization. Likewise, there is described in "Gmelins Handbuch der anorganischen Chemie", 8. Aufl., System-Nr. 27 (B), Lieferung 4 (1939), pp. 465-466, the reaction of Engel's salt (MgCO.sub.3 .multidot.KHCO.sub.3 .multidot.4H.sub.2 O) with phosphoric acid into potassium magnesium hexahydrate.
The subject of DE-OS No. 19 24 284 is also a process for the production of potassium magnesium phosphate, by reaction of magnesium oxide and potassium hydroxide with phosphoric acid.
Since the potassium salt to be employed according to these processes must first be obtained in a required process from potassium chloride, it is already recommended in DE-PS 619 397 to react a mixture of magnesium- or potassium-chloride with phosphoric acid in heat, until hydrogen chloride no longer escapes. However, these processes require apparatus of corrosion-resistant material and particular arrangements for treatment of the exhaust gas.
It is further known to produce potassium magnesium phosphate by reaction of potassium- and magnesium-chloride or -sulfate with phosphoric acid in the presence of organic amines, whereby the amine binds the acid anion liberated during the reaction, under formation of the corresponding amine salt. Such processes are already known from DE-PS No. 12 65 726, U.S. Pat. No. 3,615,186 and BE-PS No. 740 307. The use of amines in these processes has, however, an additional expenditure with regard to the recovery of the amine from the formed salts.
In order to avoid the additional expenditure, which is necessary with the known processes either for the prior obtaining of the suitable potassium compounds from potassium chloride or for the connected working-up of the amine salt, there is suggested in DE-PS No. 28 31 672 a process for the production of potassium magnesium phosphate, according to which in an acid phosphate solution with a P.sub.2 O.sub.5 -content from 20 up to 160 g P.sub.2 O.sub.5 per 1000 g water, the stoichiometrically necessary amount of basic reacting magnesium compound and a partial amount of the totally required potassium chloride, for the formation of dimagnesium phosphate, are stirred in, and into the produced suspension, after addition of the remaining amount of potassium chloride, whereby the total amount of potassium chloride to be employed is increased to a mol ratio of K.sub.2 O to P.sub.2 O.sub.5 from more than 1:1 up to a little below the saturation concentration, at a temperature of below 40.degree. C., is slowly added the amount of basic reacting magnesium compound stoichiometrically necessary for the formation of the tertiary magnesium phosphate, whereby a concentration of 130 g MgCl.sub.2 in 1000 g water cannot be exceeded in the reaction mixture.
For the basic magnesium compound which is to be slowly added around the end of these processes, it is recommended to employ a particularly reaction-active basic magnesium compound, such as for example highly active magnesium oxide or nesquehonite. Since the amount of magnesium to be added in this stage of the previously known processes in the form of the particularly reaction-active basic magnesium compound amounts to about a third of the totally necessary amount for the suggested processes, and the obtaining of such highly active basic magnesium compounds is technically expensive, the problem arises of finding a possibility which would make the employment of highly active magnesium compounds superfluous.